This invention relates to fluorescent materials, in particular fluorescent materials suitable for permitting measurement of the thickness of a layer of or comprising such materials, and methods for measuring such a thickness.
There are numerous techniques available for measurement of the thickness of a layer of material. Examples include microscopy, measurement under physical contact (e.g. by using a micrometer), beta particle backscatter measurement, photothermal radiometry, optical interferometry and X-ray fluorescence measurement. However, these methods suffer from a number of disadvantages. In many situations it is not practical to make contact with the layer of material to be measured—for example because it is moving quickly or because it might be damaged on contact. Non-contact methods of measurement generally rely on the layer of material having tightly defined physical properties that are susceptible to the measurement method, and such methods cannot be used to measure other materials. Some methods, such as X-ray fluorescence measurement, have safety risks in most situations.
One particularly demanding situation is the measurement of the thickness of a layer of material in the manufacture of sheet or web products, especially if the thickness is to be measured during production, for example to allow the production process to be controlled to maintain a desired thickness. In this situation off-line analysis (e.g. by microscopy) of samples taken from the manufacturing process is problematic because it may be difficult to take samples from the moving sheet or web; the taking of samples may damage the sheet or web; and there may be an unacceptable time lag in the off-line measurement process that prevents the measurements being used to control the production process in real time. Measurement methods that require contact with the moving sheet or web can interfere with the production process and damage the product. Non-contact methods such as beta particle backscatter measurement, photothermal radiometry, optical interferometry and X-ray fluorescence measurement suffer from problems such as: (a) the measurement apparatus being relatively expensive; (b) insensitivity at very small thicknesses; and (c) they can only be used for a limited range of coating materials. Another approach is to attempt to measure thickness by inspecting the sheet as it is travelling during the production process. However, it may be difficult to view the sheet if the sheet is travelling quickly, especially if precise measurement is needed; and as it moves the sheet may vibrate across the direction of travel.
One specific example of a situation where these problems are felt is in the production of thermal transfer foil sheets. FIG. 1 shows a cross section of a sheet of thermal transfer foil sheet. The sheet comprises a backing layer 1 of polyester film. On the backing layer is deposited a thin release layer 2 of a wax material. The thickness of the release layer is approximately 10 nm, although at that scale there is normally significant irregularity in the thickness of the layer. On the release layer is an optional dyed lacquer layer 3 for colouring and a metallised layer 4 of, for example, vacuum deposited aluminium. Finally, over the metallised layer is a thermal adhesive layer 5.
In use, the adhesive layer 5 of the film is pressed against a substrate such as a sheet of paper by a shaped hot die. This activates the thermal adhesive layer in the areas where the die presses it against the substrate, causing the sheet to adhere to the substrate in those areas. Then the die is released and the film is stripped from the substrate. The bond between the thermal adhesive layer 5 and the substrate is greater than that between the release layer 2 and the dyed layer 3. Therefore, in the areas where the adhesive has stuck to the substrate the sheet parts at the release layer 2, leaving the dyed layer and the metallised layer fixed to the substrate in those areas.
The successful operation of the hot transfer foil is sensitively dependent on the thickness of the thin release layer 3. If, the release layer is too thick then the appearance of the finished foil layer after deposition can be poor. If the release layer is too thin then when the sheet is stripped from the substrate the metallised layer 3 will not part properly from the backing layer 1, and the foil will not be deposited properly on to the substrate.
Thermal transfer foil is normally produced in a continuous web process, in which the web can run at a high speed. This high speed, together with the fact that the release layer 3 is very thin, makes it extremely difficult to measure the thickness of the release layer as the film is being produced. Therefore, manufacturers have relied on the experience of machine operators to control deposition of the release layer.
Numerous systems are known for measurement of film thickness by means of fluorescence intensity of the film.
U.S. Pat. No. 4,956,558 describes a system for determining the thickness of a fluorescent oil film on a bar rail. Blue-green laser light is focussed on to the end of an optical fibre which passes through the bar rail. Light leaving the end of the fibre illuminates the film and excites it to fluoresce. The fibre picks up a certain amount of the fluorescent light and carries it to a detector arrangement which provides a signal indicative of the thickness of the film.
U.S. Pat. No. 4,250,382 describes a system for measuring thickness of a silicone coating to which a fluorescent pigment has been added. It notes that fluorescent dyes (instead of pigments) were found to have unacceptable properties for coat weight determination.
U.S. Pat. No. 4,841,156 describes a system for measuring the thickness of a fluorescent coating on a film. The coating is illuminated with ultraviolet light, which excites the coating to fluoresce. The intensity of the fluorescent light is detected and used to determine the thickness of the coating. If the coating fluoresces at the same wavelengths as the film then an ultraviolet fluorescer can be added to the coating. The document notes that the amount of fluoresced light is linear with respect to the amount of coating present as long as the coating layer is quite thin. As the layer thickness increases the amount of light to thickness relationship becomes non-linear.
EP 0 336 029 A, WO 89/10268, U.S. Pat. Nos. 3,930,063, 3,956,630, 4,597,093, 4,922,113 and 5,717,217 disclose other arrangements for fluorescence measurement of films.
Prior art arrangements for the use of fluorescence to measure coating thickness concentrate on specific coating applications—i.e. individual coating compositions in specific circumstances where the thickness of the coating is approximately known. However, it would be useful for there to be a means whereby suppliers of coating compositions could arrange more generally for their coating compositions to be susceptible of thickness measurement by users who own suitable measurement equipment. Fluorescence intensity is generally related to film thickness by:If=ΦfIex(1−10−εct)where Iex is the excitation beam intensity, Φf is the fluorescence quantum yield, ε is the molecular absorption coefficient, c is the molar concentration of the fluorescent component and t is the film thickness. For thin films and/or low concentrations of fluorescent component, when (εct) is small, the relationship approximates to:If=2.303ΦfIexεctThus, under preferred conditions the measured fluorescence intensity is linearly proportional to film thickness. One desirable factor is that the fluorescence emission of the coating should remain a generally linear function of coating thickness over as wide a range as possible, to permit a user to measure over a wide range of thicknesses without inaccuracies due, for example, to interference from other components of the coating or from a substrate, and without a need to specially calibrate the measuring equipment to account for non-linear response. In providing a general means for various coating compositions to be measurable, the need is to provide for linearity over a wide range which is as much as possible independent of other material factors.